A well known prior sequence controller is adapted to reproduce an operation of an automatic production line in a factory in a repeated and sequential manner in accordance with a program of sequential steps of operation of a load which is previously designed by a programmer. Since a mode of description is more simplified than that required by a computer, an operation of programming is enabled by some acquirement of knowledge and as a result, a full-time programmer is not required to be employed nor must the preparation of program be ordered from an outside programmer.
Accordingly, since the program can be simply corrected in accordance with the situation of the factory, the sequence controller has come into wide use lately, and its construction ranges from a logic circuit using relays to an application of microprocessors.
A program utilizing such a sequence controller is arranged such that a relay sequence circuit is first designed and a sequence program of a descriptive mode peculiar to the sequence controller to be used in accordance with the relay sequence circuit is sequentially input into a memory means by means of an input means such as key switches so as to be stored therein.
Although such a sequence controller can be used in a simple manner as noted above, an operator is required to master logic descriptive marks such as AND or OR to form the relay sequence circuit and the way to use it. Also, the descriptive marks have variations according to the controller developers or manufacturers. Thus, without limiting the kind of sequence controller to be introduced, an operator has to master a way of programming which may be different from that of the previous programming everytime a new kind of sequence controller is introduced.
This is a burden newly applied to the operator and especially in case that an article (load) to be controlled is of simple construction, troublesome work is required relative to a simple control object, which is not acceptable to an operator.
In order to avoid such drawbacks, there is proposed such a system that one step of the program is formed by factors including:
(1) designation of output ports which output operation signals to a load connected thereto;
(2) designation of input ports to which detectors on the load are connected; and
(3) designation of time of a timer to set a time for which the output ports are opened.
A successive control program is then prepared by sequentially superposing the steps of program.
Although the sequence controller using such a simple program system has no problem in the case where the the accuracy of operation of a drive system for the means to be controlled is managed by itself or in the case that a binary ON or OFF operation for use with solenoid valves or air cylinders occurs. A time for which the steps of driving the article to be controlled are maintained is disadvantageously determined by the value previously set by the timer. For example, if the article to be controlled is a rotary drive using an electric motor, such that the control of a rotary movement is an object of the article to be controlled, a time from its rotation to its stop is sometimes different from that calculated in view of friction and inertia peculiar to the load. Although a means to detect a stop position, where a stop signal is fed back in some of the rotary drive systems have been introduced, the position where the detecting means is located has to be determined in view of the friction and the inertia.
Thus, in any of the prior art systems, the correction of errors practically requires the driving of the load.